Two HST/STIS spectra of Io spanning 1700-3100 Å\ were
obtained in Jupiterís shadow on August 18, 1999 as part of a
campaign to make coordinated HST and Galileo observations of
the Jovian system. The first observation began at 0654 UT, a
minute after total eclipse entry, and lasted 800 sec. The
second began at 0710 UT and lasted 700 sec., still in
shadow. During acquisition, immediately prior to the
eclipse, Io (diameter 1."12) was centered in the 2"x2"
aperture. The spectra were obtained with grating G230L (R
~740) in the ACCUM mode. Owing to the close proximity
of Jupiter, scattered light from the planet dominated Ioís
eclipsed spectrum and had to be removed by carefully
interpolating and subtracting the sky background in the
aperture. To confirm the validity of the weak emission
features extracted, the two observations were reduced
independently; and opposite halves of the aperture for the
combined observations were also reduced independently.
Prominent emission features were found below 2200 Å\ whose
identity is under investigation. Above 2200 Å, Ioís
spectrum rises and appears to contain a diffusely reflected
Jovian or solar continuum, perhaps sunlight refracted or
scattered by Jupiterís upper atmosphere. Emission from SO
appears to be detected, provided that Ioís atmosphere
absorbs at 2700 Å; i.e., that a spectral feature observed
at that wavelength for each observation is real. We will
discuss the possibility that this feature is an artifact. If
SO-SO2 dominates the emission below 2000 Å, excitation
by 200 eV electrons is favored over 25 eV electrons, based
on a comparison with laboratory spectra.